Ink jet recording methods are printing methods in which recording is performed by causing the jetting of small droplets of ink (ink composition), and causing these droplets to adhere to a recording medium such as paper or the like. Such methods are advantageous in that clear images with a high resolution can be printed at a high speed using a relatively simple apparatus. Ink sets used in such ink jet recording methods include ink sets comprising respective cyan (C), magenta (M) and yellow (Y) inks, ink sets in which a black (K) ink is added to these inks, and the like. For example, an ink set combining cyan, magenta and yellow inks which makes it possible to obtain good images, especially images with a good hue, in addition to possessing light resistance and water resistance, has been disclosed (Japanese Patent Application Laid-Open No. 10-120956).
In recent years, ink sets having light and dark inks which differ from each other in color density while being of the same color have been developed in order to realize both a broader range of color reproducibility and a suppression of conspicuous graininess when images are expressed by dots (a state in which the dots appear to be grainy when observed with the naked eye). For example, there are ink sets which have the four inks of C, M, Y and K as dark inks, and the four inks of light cyan (Lc), light magenta (Lm), light yellow (Ly) and light black (Lk) as light inks.
By performing recording while varying the amount of coloring material applied per unit area of the recording medium (i.e., varying the duty) using such an ink set comprising light and dark inks, it is possible to reduce the grainy sensation caused by the ink dots, and to obtain an output of color images with a high image quality comparable to that of silver salt photographs.
However, images in recorded matter formed using such conventional pigment ink sets suffer from the following problem: namely, a phenomenon in which the hue of the recorded images varies when the light source providing illumination varies, i.e., metamerism (light source dependence), occurs. For example, ink sets comprising a yellow ink containing C. I. pigment yellow 110, and ink sets for ink jet recording which provide clear, high-quality images and superior light resistance on special coated media, have been developed as techniques for reducing this metamerism. However, in cases where images are formed using such ink sets, the green color reproducibility is inadequate (Japanese Patent Application Laid-Open No. 2002-332440, Japanese Patent Application Laid-Open No. 11-228888).
Furthermore, ink sets comprising special color inks such as green inks, red inks, violet inks or the like besides yellow, magenta and cyan inks for the purpose of suppressing graininess caused by dot expression, improving the range of color reproducibility and reducing metamerism, have also been reported. For example, an ink set comprising a violet ink and red ink has been disclosed (Japanese Patent Application Laid-Open No. 2001-354886).
Such special color inks have a low lightness; however, in the case of a high resolution of 720×720 dpi or greater, since the radius of the dots applied to the media is small, graininess caused by dot expression is permissible even in cases where special color inks with a low lightness are deliberately printed in high lightness areas. However, at a low resolution such as 720×360 dpi, since the radius of the dots applied to the media is large, the graininess caused by dot expression becomes conspicuous in cases where special color inks with a low lightness are deliberately printed in high lightness areas, so that such printing cannot be allowed. Accordingly, it is difficult to achieve both a reduction in metamerism and a suppression of graininess in high lightness areas using ink sets comprising special color inks with a low lightness. In this case, it might be thought that both a reduction in metamerism and suppression of graininess in high lightness areas could be achieved by converting special color inks into high lightness inks; in such cases, however, the reproducibility of high-saturation low-lightness colors drops.
Furthermore, an increase in the image quality and stability of the gray balance are required in the printing of monochromatic images. A recording apparatus using two or three types of black inks with different pigment concentrations (Japanese Patent Application Laid-Open No. 11-48502), a recording apparatus which separately performs monochromatic output and color output with at least one type of color ink and concentration types greater than the number of colors (Japanese Patent Application Laid-Open No. 11-320924) and the like have also been proposed in order to increase the reproducibility of monochromatic images; however, these apparatuses cannot output both color images and monochromatic images in a favorable manner. Moreover, since ink jet printers use liquid inks, effects of changes in the external environment that cause the viscosity of the inks to vary may cause minute color variations. Accordingly, stability of the gray balance is required.
Conventionally, furthermore, ink sets of this type have included inks for alleviating metamerism that have colors equivalent to specified tertiary colors obtained by the mixing of the three types of primary color inks cyan, magenta and yellow (CMY), and that have a spectroscopic reflectance that is flatter than the spectroscopic reflectance of any specified tertiary color. Furthermore, at least one color in the color images is reproduced using a metamerism alleviating ink and at least one of the three types of primary color inks CMY; as a result, the colorless spectroscopic reflectance characteristics that are reproduced are made flatter than in the case of reproduction by the mixing of the three types of primary color inks CMY (for example, see Japanese Patent Application Laid-Open No. 2002-225317).
Since metamerism alleviating inks must be prepared in the conventional ink sets described above, the following problems are encountered: namely, the structure of the ink set is increased in size, and ink control also becomes more complicated. Furthermore, there is also a method in which metamerism is reduced by altering the Y ink in the combination of the CMY ink set; in this case, however, the following problem arises: namely, the color reproducibility in the green direction in high-lightness regions is greatly reduced.
Accordingly, it is an object of the present invention to provide an ink set which realizes superior color reproducibility and a reduction in metamerism.
Furthermore, it is another object of the present invention to provide an ink set, recording method and recording system that make it possible to obtain recorded images which are superior in terms of color reproducibility, and in which metamerism is reduced without any conspicuous graininess caused by dot expression, without using special color inks other than YMC inks, and in particular, an ink set which makes it possible to obtain recorded images that show no color variation caused by the application of pressure in addition to having the abovementioned superior performance values, and an ink set which is highly reliable as an ink set for use in ink jet recording, and further to provide recorded matter having superior recorded images that have good color reproducibility and reduced metamerism without conspicuous graininess caused by dot expression.
Furthermore, it is still another object of the present invention to provide an ink set and recording apparatus which can output both monochromatic and color images with a high quality, which make it possible to obtain recorded images that are superior in terms of color reproducibility and that have reduced metamerism without conspicuous graininess caused by dot expression, even without using special color inks other than YMC inks, and which make it possible to obtain a stable gray balance.
Furthermore, it is still another object of the present invention to provide an ink set, ink jet recording method and ink jet recording apparatus which make it possible to alleviate metamerism by means of a simple construction without any great reduction in the region of color reproduction.